1. Field of the Invention
This invention relates to methods of thermal plastic working of metal materials.
2. Description of the Prior Art
It is well known that, when a metal material undergoes metallographical changes during processing, the phenomenon called "superplasticity" may present itself to provide the possibility of an extremely large plastic working of the material, and the method of plastic working utilizing this nature has been introduced in an industrial scale.
The above mentioned introduction originated with the invention of an improved method of plastic working of metal material by the inventors herein, followed by the utilization thereof in a factory of which the inventors are in the service, which method comprises the steps of:
a. effecting changes in the temperature of a material to be plastically worked;
b. determining variations in the metallographical condition of the material caused by the changes in the temperature of the material;
c. detecting a sudden change in the metallographical condition of the material caused immediately before the material becomes superplastic; and
d. starting a plastic working of the material after having detected the sudden change in the material.
On the other hand, it has been well known in the art that, when a metal material undergoes metallographical changes during processing, the phenomenon called "superplasticity" may present itself to provide the possibility of an extremely large plastic working of the material. However, the temperature of metal material causing it to start changing in its metallographical condition depends not only upon its constituent elements to a slight degree, but also upon its history of heat treatment or other kind of processing and its rate of heating or cooling until the foregoing temperature may be reached. Also, when metal materials are heated or cooled for a relatively shorter period of time in an industrial scale, the conventional method of measuring the material temperatures during treatment is subject to such disadvantages as delays or errors in measurement; that is, with the conventional method, it is not easy to maintain the uniform conditions of measurement of the material temperature. For example, when the temperature of metal materials is measured by using a radiation pyrometer, the rate of radiation to the pyrometer may be varied according to the particular surface condition of the material. Also, when such a measurement is made by using a thermocouple-type thermometer, the measurement may be effected by the particular way of connection of the thermometer to the material.
Moreover, the temperature range of metal material producing the condition of superplasticity is relatively smaller; therefore, when the material has reached such a temperature, it is not easy to start a plastic working thereof, without losing time, so as to obtain a desired product. This difficulty has prevented the nature of superplasticity of metal material from being fully utilized in the plastic working thereof in an industrial mass production.
The difficulty of plastic working of metal utilizing its superplasticity may be described in more detail. Firstly, the temperature of metal material causing it to become superplastic varies according to the particular kind of the material, or the particular chemical composition thereof. In addition, even metal materials with the same chemical compositions may differ from one another in the foregoing temperature if they have different histories of processing. Thus, with a factory where a great number of metal materials are to be plastic worked, it is practically impossible in actuality to determine the "superplasticity temperature" of every individual material which may be different from the others in its chemical composition and/or processing history.
Secondly, if the "superplasticity temperatures" of metal materials are experimentally determined in advance by using test pieces, the temperature value so determined may not be reliable enough to ensure that the actual plastic working of the material can be made exactly when it has certainly reached its real "superplasticity temperature". The reason for this is not only that the determinations are subject to the inherent restriction that the temperature range of a metal material causing it to become superplastic is narrow (as described before), but that the (conventional) determinations themselves are not free from delays or errors (as described before). Hence, in no small case, such an experimentally determined value is not the real "superplasticity temperature" of the material. It may still be extremely difficult, therefore, to make a plastic working of the material, even with a prior attempt at detecting the foregoing temperature, exactly when the material has reached it.
From the foregoing reasons, in general it has been difficult to utilize the superplasticity of metal material for the plastic working thereof in an industrial scale and no company has yet introduced this technique for the above mentioned purpose, except the factory of which the inventors herein, who have developed the improved plastic working method as described before, are in the service. In addition, it seems that no company except the inventors' factory has yet had even the idea of utilizing superplasticity of metal for the plastic working thereof.